Abstract—In this paper, experimental study on drilling of

multi-material made of CFRP laminate sandwiched with

aluminum part has been carried out. These tests have been

conducted using carbide drills (K20) to study the influence of

spindle speed, feed rate and lip length of the double cone drill on

cutting forces and holes quality. From the experimental study it

was found that the double cone drills used for the experimental

analysis encountered less thrust force compared to the standard

twist drill. In addition the quality of the holes after drilling were

evaluated, and found that no delamination (entry and exit of the

hole of the composite part) even at higher feed rate (> to 0.1

mm/rev). From the SEM observation it was noticed that, after

machining with standard twist drill, several damaged areas

were observed on the wall of the holes.

Index Terms—Drilling, multi-stack, Damages, tool design,

aluminium chips.

I. INTRODUCTION

Drilling and fastening of the hybrid materials in one-shot

operation reduces manufacturing time. One of the most

common problems encountered during automatic drilling of

sandwich structures are that the continuous chips and high

temperature generated during drilling of Ti, and damage in

composites. LATECIS a French company in Toulouse,

developed a system called OPERA, dedicated for one shot

drilling. Ideally the OPERA system is expected to handle

high production rates (1 attachment mounted approximately

at every 12 seconds) [1], [2]. The automation of these tasks

must also enable greater mounting precision, improved

ergonomics, health and safety of the operators, particularly

for the new hybrid materials like composite/titanium or

composite/aluminum assemblies. Due to the different

mechanical properties of materials constituting the hybrid

panels, their machinability remains an open problem and can

prevent the automation of these tasks. From the literature

review, it is found that composite materials especially Carbon

Fibre Reinforced Plastics (CFRP) are often used to a larger

extent due to their downsizing and rightsizing impacts.

However, due to their laminated structure and the anisotropy

of the carbon fiber reinforce plastics several damages are

seen during drilling. The damages are peel up delamination at

the hole entry, thermal alteration, fibre pull-out and fuzzing

on the wall of the hole, and exit delamination and uncut fibre

at the hole exit [3]-[5]. Moreover, the mechanism of material

removal in composite materials is strongly influenced by the

relative angle between the direction of the cutting speed and

Manuscript received January 10, 2014; revised March 26, 2014.

The authors are with the Mechanical Engineering Department, Paul

Sabatier University (University of Toulouse, France), France (e-mail:

Redouane.zitoune@iut-tlse3.fr).

the direction of fiber [6]. Sakuma et al. [4] drilled holes using

four drill materials and investigated drill wear pattern, flank

wear width and cutting forces. Many researchers investigated

the effect of tool geometry on drilling of polymer composite

materials. Many of the modified geometries (Zhirov point,

Brad and Spur, step drill, candle stick, saw drill, etc) are

difficult to regrind [5]. Krishnaraj et al. [7] drilled composites

at high spindle speed and studied the influence of tool

geometry. They reported that double cone drill offers better

surface finish when compared to standard twist drill and Brad

& Spur drill. In particular double cone drills which the length

of the principal cutting edge is equal to the length of the

secondary cutting edge offer better finish [7], [8].

Unlike composites, the material removal during machining

of aluminium is mainly done by shearing the material. In the

case of drilling of aluminium and its alloys, the main problem

is the adhesion of aluminium on the main cutting edges

(BUE), on the rake face and on the flutes of the drill (BUL).

This bonding is responsible for premature wear of the cutting

tool, for the poor surface finish of the hole and the variations

in the diameter of the hole. Several authors showed that, with

low cutting speeds (up to 25 m/min), bonding of aluminium

occurs at the rake face and at the main cutting edges [9]–[11].

One way to overcome problems related to the machining of

aluminium and its alloys is to increase cutting speeds.

However the machining with very high cutting speeds (Eg.

300 m/min) causes a significant increase in the cutting

temperature (above 300 °C) [11]. At this temperature a

chemical reaction between aluminium and cobalt occurs to

form micro welding of aluminium on the cutting edges of the

tool by diffusion. In the experimental works of [12], [13], the

authors during the dry turning of aluminium alloy have

shown that, in the first time of machining, BUL is caused by

thermo-mechanical mechanisms. Once the BUL is formed,

the initial cutting conditions change thus enabling the BUE

formation through mechanical adhesion. However, during

continuous machining, the BUE continues to grow until a

critical thickness is reached, and once it reaches the point, it is

plastically extended over the BUL due to the action of the

mechanical forces.

Additionally, the chips shape and length of the chip

passing through the hole as well as built up edges of

aluminium at the primary cutting edges combined with

increased tool wear affect the hole quality [1], [2], [14]. For

machining multi-material, sharp and high hot hardness tool

materials are required. [15], [16] reported that tool wear

occur rapidly when drilling Gr/Bi-Ti stacks. Multimaterial

theoretically calls for different tools, one that fits the

attributes of a composite and a different one that fits the

attributes of aluminium, or Titanium. Multilayer materials

drilled with adapted step drills improved diameter tolerances,

Influence of the Double Cone Drill Geometry on the Holes

Quality during Drilling Multi-Stack Made of CFRP/Al

R. Zitoune, N. Cadorin, B. S. Elambouacif, F. Collombet, V. Krishnaraj, and H. Bougherara

International Journal of Materials, Mechanics and Manufacturing, Vol. 2, No. 4, November 2014

292DOI: 10.7763/IJMMM.2014.V2.145

surface quality, and reduced tool wear [14].

Ref. [15] and [16] have studied drilling of Gr/Bi-Ti in the

context of process conditions and cost optimisation.

Among the various tool geometries investigated, double

cone drill was found to offer many advantages when

compared to the modified geometries. Only a few

investigations on drilling of CFRP laminates using double

cone drill have been reported. In literature, the double cone

drills are optimized for drilling of metallic materials (steel or

aluminum). However, there is little information about the

influence of the geometry of the double cone drill on the

quality of holes machined. This study attempts to achieve this

objective. In this paper, experimental study on drilling a

hybrid material made of CFRP laminate aluminum part has

been carried out using carbide drills (K20) to study the

influence of spindle speed, feed rate and lip length of the

double cone drill on thrust force and hole quality.

II. EXPERIMENTATION PROCEDURE

A. Workpiece Details

The sandwich plates studied are composed of a

carbon/epoxy and aluminium. The CFRP composite

specimen used in the investigation was 4.2 mm thick. The

laminate was made out of 16 unidirectional plies of 0.26 mm

thickness each. The 16 unidirectional plies are made of

carbon/epoxy prepreg and manufactured by Hexcel

Composite Company with the reference T700-M21. The

following was the staking sequence [90/45/0/-45]2s. These

materials were compacted using a vacuum pump in a

controlled atmosphere. A mould for the laminate was

prepared and placed in a vacuum bagging and evacuated to

0.7 bar (see. Fig. 1a). Curing was then carried out at 180 °C

for 120 min during which the pressure was maintained at 7

bar in an autoclave. Fig. 1b shows the laminate used for

conducting the experiments. The aluminum alloy used in this

study is extensively used in aviation industry have referenced

as Al 2024. The percentage of alloying elements is as follows:

Al 93.5% Si 0.5%, Cu 3.8–4.9%, Mg 1.2–1.8%, 0.1% Cr.

Fig. 1. Manufacturing of the composite part by the autoclave process.

B. Drilling Details

Drilling is done on a CNC machine developed by

LATECIS Company (Toulouse, France) under the research

project OPERA (Automated drilling and riveting of aircraft

structures) is shown in Fig. 1a. The acquisition of cutting

forces is carried out using a four-component Kistler

dynamometer as shown in Fig. 2a. The dynamometer is

connected to a Kistler charge amplifier type 5019. The output

of the amplifier is transformed into a cutting force through a

computer that stores the force signals versus cutting time.

The sandwich panel to be drilled is clamped on a dedicated

support (see Fig. 2b). On the latter, a hole of 18 mm is

machined to allow the drill bit and to prevent the bending of

the sandwich plate. The drilling tests performed are based on

full factorial experimental design using three feed rates (0.05

mm/rev, 0.1 mm/rev and 0.15 mm/rev) and two spindle

speeds (2020 rpm and 2750 rpm). Characteristics geometric

of the reference tool (ref-tool) marketed by French industry

for drilling of composite materials for Air-Bus industries is

presented in Fig. 3a, and the same has been modified to

double cone geometry. The modification of the tool geometry

is achieved using a 5-axis grinding machine.

Fig. 2. Experimental device for drilling tests. With: (a) Front view of

OPERA system, (b) Zoom of the area surrounded area of (a).

Fig. 3. CAD models showing the active portion (point) of the drilling tools

with (a) reference tool (b) double cone tool.

The double cone tools measure 90° and 132° point angles

with cutting edge lengths, namely L1 and L2 (see Fig. 3)

while L1 represents the size of the principal cutting edge

number 1 which is characterized by a point angle of 132°, L2

represents the size of principal cutting edge number 2 which

is characterized by a point angle of 90°. The double cone

drills tested are ground with different L1/L2 ratios, in which

M1, M2 and M3 represent L1/L2 ratios of 0.33, 1 and 3.1

respectively. Drilling trials were carried out using a 6.35 mm

of diameter made of tungsten carbide (K20). For the

confidentiality reasons is not possible to give more details

about the geometry of the tools. Each experimental condition

was repeated 5 times in order to get consistent values. To

remove the influence of tool wear, each experiment was

performed with a new drill.

The quality of the machined surface (wall of the hole) is

quantified using the SEM observation and the surface

roughness. The surface roughness (Ra) of the hole was

measured by surface roughness tester (Mitutoya SJ 500) with

a sampling length (cut-off) of 0.8 mm. In case of CFRP the

length of measurement through the hole was 4 mm (0.8 × 5 =

4 mm).

III.

RESULTS AND DISCUSSION

A.

Cutting Force Analysis

Fig. 4 represents the evolution of the thrust forces during

Vacuum

Sensors to monitor

temperature Rigid mould

International Journal of Materials, Mechanics and Manufacturing, Vol. 2, No. 4, November 2014

293

drilling with reference drill, double cone drill M2 and for

machining parameters of 0.05 mm/rev of feed rate at a

spindle speed of 2020 rpm. From this figure it is noticed that,

the increasing of the second cutting length leads to reduce the

thrust forces. If we plot the average value of the cutting forces

for all tested tools and for various feed speed, it is observed

that, drilling with the reference tool experiences higher thrust

force compared to the double cone tools (see Fig. 5).

When the feed rate has been increased from 0.05 mm/rev

to 0.15 mm/rev, the drilling with the reference tool is

subjected to an increase of 60 % of the total thrust force in the

composite (with the maximum value of 126 N) and 158 % in

the aluminum (with the maximum value of 481 N). This

result observed while drilling at a spindle speed of 2020 rpm.

Similar increase of thrust forces are observed when drilling

with double cone tools as well. However, for the same

machining parameters, drilling with double cone tools

produce 15 % to 30% lesser thrust force in the composite

when compared to the thrust produced when machining with

a reference tool. This can be explained by the fact that adding

a secondary point angle (90°) reduces the theoretical average

chip thickness by 15 %. From the literature works focused the

orthogonal cutting on unidirectional composite material;

various authors have shown that, the increasing of the depth

of cut (chip thickness) lead to increase of the cutting forces.

During drilling at a spindle speed of 2750 rpm, similar results

are obtained compared a spindle speed of 2020 rpm.

The evolution of the thrust force generated during drilling

of aluminum vs. the feed rate show that, the level of the

recorded forces induced by the reference drill and the double

cone drill M2 and M3 are similar. However, the double cone

drill M1 induces a less thrust force. This difference can be

attributed to the fact that, adding a secondary point angle (90°)

on the one hand reduces the theoretical average chip

thickness and on the other hand increase the surface contact

between the drill bit and the aluminum part. This increasing

of the surface contact leads to increase the friction. This can

be confirmed thanks to torque level recorded vs. the feed

rates and tool geometries (see Fig. 6).

Fig. 4. Thrust force evolution during time of drilling for different tool

geometries.

Fig. 5. Influence of the tool geometry and feed rate on the thrust force during

drilling at a spindle speed of 2020 rpm. (a): thrust force in the composite, (b):

thrust force in the aluminum.

Fig. 6. Influence of the machining parameters and the tool geometry on the

torque during drilling aluminum.

B. Quality of the Machined Holes

Fig. 7 and 8 show the effect of drill geometry on surface

finish at various feed rates, for a spindle speed of 2020 rpm in

the CFRP and aluminium respectively. Experimental results

can reveal that at a low feed rate (< 0.1 mm/rev), the quality

of the machined surface is better for any drill used. In this

case, values of the measured roughness are smaller (< 3 µm).

Further it can also be observed that, for all machining

parameters used, values of the surface roughness obtained

with the reference tool are inferior when compared to those

obtained with double cone tools. This difference can be

linked to the interaction between the size of chip thickness

and the drill point angle. In addition, double cone drills

favour the apparition of continues chips during drilling of

aluminium. For this small roughness are obtained on the wall

of aluminium holes when drilling is conducted with double

cone drills compared to the reference drill.

Fig. 7. Evolution of the roughness in the CFRP vs. feed rate for reference

drill and double cone drills at a spindle speed of 2020 rpm.

Fig. 8. Evolution of the roughness in the aluminum vs. feed rate for reference

drill and double cone drills at a spindle speed of 2020 rpm.

With the increasing if the length of second edge of the

double cone drills we reduce the chip thickness and we

improve the machining quality (see Fig. 8). Drilling with

double cone drills offer stable surface roughness (inferior to

0

0.2

0.4

0.6

0.8

1

1.2

0.05 0.1 0.15

To

rqu

e (N

.m)

Feed rate (mm/rev)

Al - reference drill

Al - double cone - M1

Al - double cone - M2

Al - double cone - M3

0

1

2

3

4

5

6

0.05 0.1 0.15

Ro

ug

hn

ess R

a (

µm

)

Feed rate f (mm/rev)

CFRP - Reference drill

CFRP - Double cone - M1

CFRP - double cone - M2

CFRP - double cone - M3

0

1

2

3

4

5

6

0.05 0.1 0.15

Ro

ug

hn

ess R

a (

µm

)

Feed rate f (mm/rev)

Al - Reference drill

Al - Double cone - M1

Al - Double cone - M2

Al - Double cone - M3

International Journal of Materials, Mechanics and Manufacturing, Vol. 2, No. 4, November 2014

294

1µm) in the aluminium when machining is conducted with

feed rates between 0.05 and 0.1 mm/rev and for any spindle

speed (2020 rpm or 2750 rpm). However in the same

machining parameters machining with reference drill gives a

roughness values superior to 1 µm. In addition, for all tools

used, drilling with 0.15 µm, the roughness measured in the

aluminium is superior to 1 µm.

(a)

(b)

(c)

Fig. 9. Influence of the feed rate on the aluminum chips after drilling with

double cone drill M2. With: (a) N = 2020 rpm, f = 0.05 mm/rev, (b) N = 2020

rpm, f = 0.1 mm/rev, (c) N = 2020rpm, f = 0.15 mm/rev.

Fig. 10. Different nature of damage observed when drilling is conducted with

small feed rate (0.05 mm/rev, spindle speed of 2020 rpm) and for all the

tested drills. (a): Chips flow, (b): damages on the wall of the CFRP hole.

Generally, drilling with double cone drill M2 give better

machining quality. Specially, drilling with a feed rate of 0.1

mm/rev and using the double cone tool (M2) offers a small

roughness at both the spindle speeds in the composite and

aluminium holes (roughness in the CFRP inferior to 3 µm and

in the aluminum inferior to 1 µm). In addition drilling with a

feed rate of 0.1 mm/rev favors the apparition of broken

aluminum chips. The stable surface roughness value beyond

0.1 mm/rev could be due to the wiping effect of the primary

cutting edge 2.

It is important to mentioned that, the drilling with a small

feed rate (0.05 mm/rev) favors the apparition of continues

chips for all used tools (see Fig. 9). With these continues

chips two problems have been observed when the OPERA

system is used. The first problem is the interaction of these

chips with the composite, leads to the degradation of the first

ply of the CFRP part located at the hole entry as well as the

damages apparition on the wall of the hole (see Fig. 10).The

second problem is the aspiration system malfunction of

carbon dust.

IV. CONCLUSION

From the experimental study carried out during drilling of

multi-material made of CFRP/Aluminium, using different

double cone drills, the following conclusions can be drawn:

Drilling using double cone drill presents, lesser thrust

force when compared to drilling using the reference drill.

Especially the drilling using double cone drill (Type M2)

resulted in reduced average roughness values when

compared to other double cone drills and reference drill

at 0.1 mm/rev feed rate. Also when drilling an isotropic

material such as steel or titanium with double point

angle, it has been recommended [34] that the ratio of the

second cutting edge to the principal cutting edge (L2/L1)

is to be equal to 0.15. One can conclude that it is

suggested to use double cone drills with L2/L1 equal to 1

when drilling CFRP materials.

No push-out delamination was observed even at higher

feed rate above 0.1 mm/rev. This can be attributed to the

aluminium plate at the bottom of the laminate and also to

the presence the small thrust forces.

The presence of continues chips (aluminium) at low feed

rate impact the hole quality of the composite by the

presence of the peel up delamination at the top of the

hole.

ACKNOWLEDGMENT

The author’s sincerely thank Midi-Pyrénées Region

(French) for their financial support.

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Redouane Zitoune was born in 1976. He is an

associate professor in mechanical engineering at Paul

Sabatier University (University of Toulouse, France), since 2005. The PhD work of Redouane Zitoune is

focused on machining (drilling and milling) of

composites materials. His current research interests include damage analysis during drilling and milling of

composites materials (with conventional machining

and abrasive water jet machining) and finites elements simulation of machining. He is also interested in the thermal analysis of composites

structures by using an optical fibres and finite element analysis. He has

published more than 60 technical papers in National and International Journals/conferences. In the area of machining of composites materials he

has organized the first national meeting in May 2012. This scientific event

has been organized with the collaboration of the French Aerospace Lab (ONERA) and with the consent of the national Association for Composites

MAterials (AMAC).

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